Regenerating scintillation counter



July 21, 1959 J, LEMPERT REGENERATING SCINTILLATION COUNTER Filed Nov.26. 1954 44 .V VLM v fw? /LVL .....t n. FQQQQQQQQQQQQQ QDNQQQQQQQQQQDNWNmvENToR Joseph Lemper.

WITNESSES TTORNEY United States Patent O 2,896,088 REGENERATINGSCINTILLATION COUNTER Joseph ljempert, Elmira, NY., assigner toWestinghouse Electric Corporation, East Pittsburgh, Pa., a corporationof Pennsylvania My invention relates to radiation detectors and inparticular to a regenerative form of radiation detector which is ofgreater sensitivity than radiation detectors of the prior art. It alsoincludes a novel type of radiation image intensier and is alsoapplicable to a novel type of current pulse generator.

Radiation detectors have been proposed in which radiation quantaincident on a suitable phosphor or other uorescent material generatelight which strikes a photoelectric cell and causes current fiow in theoutput circuit thereof. In accordance with one feature of my invention amore sensitive radiation detecting device is attained by forming thephotoelectric cathode as a thin layer parallel and closely spaced fromthe fluorescent layer, for example by superimposing the two layers onopposite sides of the thin glass wall of an electronic tube, andbombarding an electron phosphor output screen in the tube with thephotoelectrons generated at the surface of the photoelectric layer. Thelight flash produced in this way at the output phosphor by eachradiation quantum striking the input phosphor is of many times theenergy of the latter, and relatively weak incident radiation may be thusdetected. Moreover, where the input radiation is in the form of an imagefield, both the electron stream and the light field generated on theoutput screen may be made to reproduce the space distribution of theimage field, and the device thus produces an intensified light image ofthe input radiation field. By making the space between the 2,896,088Patented July 21, 1959 ICC . 2 electric surfaces which confine thebackward-emitted light rays to paths along these light-tunnels to thephoto electric surface.

When using the tube of my invention as a radiation detector, theregenerative properties I have described would cause a continued buildupof output light and of electron flow which could persist even aftertermination of the initiating radiation pulse unless provision were madeto terminate this buildup. The buildup may be terminated, for example,by allowing the electron flow through the photoelectric screen to chargea capacitor in the grid circuit of a` tube controlling the voltagesupply to the output of the radiation detector and thereby interruptingfurther operation of the detector until another photon of radiationbeing detected strikes the input screen.

The electron ow from the photoelectric screen would build up insuccessive steps due to the above described regenerative action; viz, agiven incident photon would cause emission of a definite number ofelectrons immediphoto electric layer and the output phosphor very small,v

and using as the electron phosphor a material which emits lightpredominantly of a wave length to which the photoelectric layer iscopiously responsive, the light emitted back toward the photoelectriclayer when its electrons strike the electron phosphor actsregeneratively on the former to generate additional electrons whichwill, in turn, be accelerated into contact with corresponding portionsof the electron phosphor output screen and increase the luminosity ofthe latter still further. By this regenerative action, the sensitivityof the apparatus to the input radiation is greatly increased.

If the electrons projected from the photoelectric surface can becompelled to traverse paths normal to the screen surface, and the lightprojected backward from the electron phosphor output surface could beconfined to similar normal paths, the distribution of light intensity inthe output image would correspond exactly to the distribution ofintensity of the input radiation, and the light image on the outputscreen would be an exact replica of the input radiation field.Scattering of electrons from such normal paths may be effectivelyprevented by providing a magnetic field normal to the screens; and theeffects of both electron scattering and light-ray scattering may beminimized by making the distance between the photoelectric surface andthe electron phosphor surface' small. Furthermore, scattering of theback-reflected light may be minimized by providing a grid of thinpolished metal strips forming a kind of honeycomb of fine-bore lighttunnels between the electron phosphor and photoately following itsincidence; after these had had time to travel Vto the electron phosphorscreen they would generate light, and as soon as the latter traveledback to the photoelectric screen a second burst of electrons would beadded to the electron stream. The length of this interval betweeninception of the first and second electron pulses from the photoelectricscreen could be given any desired value by properly fixing the voltageand spacing between the photoelectric screen and the electron phosphorscreen; and by arranging a capacitor circuit in a well-known way to cutoff current flow after occurrence of a predetermined number of suchpulses, a novel type of pulse generator is obtained.

One object of my invention is accordingly to provide a novel and moresensitive type of radiation detector.

Another 'object is to provide a novel circuit for radiation detectorsmaking use of a regenerative combination of photoelectric screens andelectron phosphor screens.

Another Objectis to provide a novel type of periodic electrical pulsegenerator.

Another object is to provide a novel and improved typeV of radiationimage reproducer.

Another object is to provide a novel and improved type of radiationfield transformer.

Another object is to provide a new type of light or other radiationimage intensifier.

Yet another object is to provide a novel arrangement for improving theresolution and preventing blurring of the images in picture reproducingtubes which utilize combinations of photoelectric andelectron phosphorscreens.

Other objects of my invention Will become apparent to those skilled inthe electronic art upon reading the following description'taken inconnection with the drawings in which the figure is a schematic view inperspective of a tube in accordance with my invention which may beemployed either as a radiation detector or as a radiation imagereproducer.

Referring in detail to the drawings, a radiation detector and imagetransforming tube embodying the principles of my invention may comprisea vacuum-tight enclosure 1 of glass having a pair of flat parallel sidewalls, one of these bearing on its outer surface a layer or input screen2 of material which the radiation to be detected causes to emit light orother radiation. For example, if the radiation to be detected is X-raysthe layer 2 may comprise zinc sulphide. The side wall bearing layer 2should be as thin as is practicable and should have its inner facecoated with a thin layer 3 of a material which l current flow fromexternal circuits to layer 3. Parallel v to photoelectric layer 3, andspaced away from it by the minimum distance alfording insulation againstthe voltage impressed between them, is an output layer or screen 5 whichmay, if desired, be supported on the opposite side wall' of container 1and which comprises. an electron phosphor which preferably emits lightor other radiation to. which layer 3 is photoelectrically responsive.-To give one instance, layer 5 may be ZnSiO4'zMn or Zn2SiO4zSi. A lead-in6 makes it possible to impress voltage on output screen 5.

While operability of this tube is not dependent upon its presence,performance as a transformer or reproducer of input radiation imageswill be better if the space between layers 3 and 5 is occupied by a gridor honeycomb V7 of thin polished sheet-metal strips forming light ductsor tunnels normal to output screen 5 and having 'diameters smallrelative to their lengths so that light generated by impact of electronsat screen 5 is prevented from scatter'- ing laterally from a path normalto that screen in traveling toward photoelectric layer 3. The honeycomb7 is provided with a lead-in 8 by which its potential may be fixed atwill relative to photoelectric layer 3. If the potential of thehoneycomb 7 is made sufficiently positiveY to layer 3, secondaryemission may be attained of such magnitude as to amplify the electronstream by secondary emission.

The lead-in 4 for photoelectric layer 3 is connected through a pulsecounter 11, of any known type capable of registering the number ofpulses of electric currentY passing through it, to the negative pole ofa direct-current voltage source 12 having its positive pole connectedthrough a resistor 13- and an inductance 14 to the anode of a`grid-controlled electron tube 15. The cathode of tube 15 is connectedthrough the lead-in 6 to output screen 5. A direct-current bias source16 makesit possible to impress a desirable bias between honeycomb 7 andphotoelectric layer 3. A capacitor 17 shunted by bleeder-resistor 18 isconnected through a pair of blocking capacitors 19 across the resistor13; and the terminals of capacitor 17 are connected to the controlelectrode and cathode of tube 15 so that, as charge accumulates on thecapacitor when current flows through tube 15, the control electrode ofthe latter becomes increasingly negative relative to its cathode andalternately blockscurrent flow therein. Any other blocking tube circuitknown in the art as suitable to block current flow after, passage of apredetermined charge may replace this tube 15 circuit, however.

The above-described circuit may be used as. a radiation detector in thefollowing way. The radiation toy be detected is projected onto the inputlayer 2 where each photon generator' projects light from its point ofimpact, through the wall of enclosure 1, and onto the adjacent smallarea of photoelectric layer 3. Electrons are emitted from the oppositeface of this small area and are attracted by the electric field duetothe positive potential impressed by voltage source 12 through controltube 15 on output screen 5. The electrons from the above-mentioned smallarea pass through the particular tunnel or duct of honeycomb 7 whichoverlies that area, and are thus compelledto move in paths normaltofphotoelectric screen 3 and prevented from scattering laterally toother portions of output screen 5.

Upon striking output screen 5,` the electrons generate light, part ofwhich is projected forward to points outside tube 1, and part of whichis projected backward, through the above-mentioned light duct inhoneycomb 7, onto the same small area of photoelectric screen 3 in whichthe electrons originated. The light duct in honeycomb 7 has thusprevented any scattering of either electrons or the returning light fromthe particular small area of theinput screen on which the originalradiation photon was'incident.

The light returnedto photoelectric layer 3* causes the emissiontherefrom of an additional burst` of electrons which are attractedthrough the same light duct of honeycomb 7 into incidence with the samearea of output screen 8; there they emit more light to points outsidetube 1 and return more light to photoelectric screen 3 where theygenerate a third burst of electrons. This cumulative buildup bysuccessive steps of electron ow from photoelectric screen 3 and of lightemission from output screen 5, at the small area corresponding to thepoint on the input screen where the detected radiation photon wasincident would continue indefinitely, and long after such incidence,were this regenerative action not terminated by some means, such asinterruption of the impression of voltage output on screen 5 throughcontrol tube 15. To eect such an interruption of the regenerative actionresistor 13, which is traversed by the current corresponding to electronflow between photoelectric screen 3 and output screen 5, is shunted bycapacitor 17 so that the latter acquires a charge corresponding to eachburst of electrons passing through the duct of honeycomb 7. Capacitor 17is so connected to the cathode and control grid of control tube 15 as toimpress a negative bias thereon which grows greater with each successivecumulative step in the buildup of the electron ilow to output screen 5.This step-by-step increase of the negative bias tends to terminatecurrent flow through control tube 15, but is opposed for a time by theself-inductance of inductance 14. Soon, however, the current throughtube 15 sinks to zero, the effectiveness of inductance 14 expires, andthe acquired negative charge on capacitor 17 keeps control tube 15nonconductive until that charge leaks away through bleeder-resistor 18.After an interval iixed by the time constant of capacitor 17 andresistor 18, control tube 15 becomes conductive again, and the systemcan react as described above to the incidence of a new photon of thedetected radiation on input screen 2.

To obtain proper operation of this apparatus as a radiation detector andphoton counter, the time period of the capacitor 17-resistor 18 circuitmust be less than the time intervals between incident photons of thedetected radiation, as must also be the decay-period of the phosphors inscreens 2 and 5. The pulse counter 11 will then register each pulse ofcurrent built up through control tube 15 and so register the number ofphotons incident on input screen 2.

The time between successive steps in the buildup of current produced byeach incident photon is the time required by an electron to traverse thedistance between photoelectric screen 3 and output screen 5 plus thetime required for light to return to screen 3 over the same distance,the latter time being usually negligible compared with the former, andbeing adjustable by varying the voltage impressed by source 12. Thecircuit may thus be considered as a means for generating stepped currentpulses of predeterminable period, and comprising on each pulse a numberof steps regulated by properly adjusting the time-constant of thecapacitor 17resistor 18 circuit.

Since the honeycomb 7 insures that the illumination produced on outputscreen 5 is confined to a small area corresponding in position to thatone which the incident photon of the detected radiation strikes, aspace-dis tributed eld of detected photons will produce a lightemissionon output screen 5 which is a replica in distribution of that field; inother words, my arrangement will transform the input radiation fieldinto an output radiation iield from screen 5, which is its replica orimage. T hus, for example, a picture will appear on output screen 5 ofan X-ray image which was projected onto input screen 2; and itsintensity may be many fold that of the X-ray image, being determined byenergy drawn from the local voltage source 12. In short, the tube 1 actsas an image intensifier.

The surfaces of the light ducts in honeycomb 7 may be coated withcesiated antimony .or other secondary electron emitter to increase theefficiency of this portion of tube 1. In such case the potential ofhoneycomb 7 should be suhciently positive relative to layer 3 to beabove the first crossover in the voltage vs. secondary emission curvewell known in the radio art. However, provided the distance betweenlayers 3 and 5 is made of the order of the areas in the image fieldwhich it is desired to resolve optically, the arrangement is operativeas a picture reproducer even though honeycomb 7 is omitted. A spacing ofhalf a millimeter will suffice for many images. It may be noted thatcesiated antimony is also a good photoelectric emitter and that lightwaves radiated by the output screen back into the honeycomb cells willaugment the electron stream. Silver magnesium (AgMg) is also a goodcell-lining material.

When it is desired merely to reproduce and intensify a light image theinput screen 2 may be omitted and the light image be focussed on thephotoelectric screen 3.

In reproducing image fields on the output layer 5, it is of coursenecessary that the electron density at different points over the area ofscreen 5 shall correspond to the light-intensity at corresponding areasof input screen 3. If the regenerative action described above forradiation detector operation is employed in the apparatus and allowed tocontinue too long, it will build up to the point Where theelectron-emitting power of the portion of screen 3 within each of everyhoneycomb cell will reach saturation so that further increase ofemission is impossible, and all contrast of brightness between differentareas of the output screen be destroyed. Hence the control tube i5 mustbe adjusted to interrupt the impression of voltage on screen 16 beforesaturation conditions are reached even in the brightest areas of thepicture.

I claim as my invention:

l. A radiation responsive device comprising a vacuum tight enclosure, afirst screen therein responsive to emit electrons upon incidence of aphoton of input radiation thereon, a second screen therein responsive toincidence of electrons to emit an output radiation to which said firstscreen is photoelectrically responsive, a honeycomb structure of ductspositioned between and normal to said first and second screens, and acircuit connecting said first screen to said second screen andresponsive to the amount of current flow and containing means tointerrupt current ow after passage of a predetermined amount of current.

2. A radiation responsive device comprising a vacuumtight enclosure, afirst screen therein responsive to emit electrons upon incidence of aphoton of input radiation thereon, a second screen therein responsive toincidence of electrons to emit an output radiation to which said firstscreen is photoelectrically responsive, said first and second screensbeing spaced apart by a distance of the order of resol-ution required inpictures, and a circuit connecting said first screen to said secondscreen which is responsive to the amount of current flow and containingmeans to interrupt current iiow after passage of a predetermined amountof current.

3. A radiation responsive device comprising a vacuumtight enclosure, afirst screen therein responsive to emit electrons upon incidence of aphoton of input radiation thereon, a second screen therein responsive toincidence of electrons to emit an output radiation to which said rstscreen is photoelectrically responsive, a honeycomb structure of ductspositioned between and-normal to said first and second screens, and acircuit connecting said first screen to said second screen andcontaining means to interrupt current fiow therein upon passage of apredetermined quantity of electricity, and a pulse counting device insaid circuit.

4. A radiation responsive device comprising a vacuumtight enclosure, afirst screen therein responsive to emit electrons upon incidence of aphoton of input radiation thereon, a second screen therein responsive toincidence of electrons to emit an output radiation to which said firstscreen is photoelectrically responsive, and a circuit connecting saidfirst screen to said second screen and containing means to interruptcurrent ow therein, and a pulse counting device in said circuit.

5 A radiation responsive device comprising a vacuumtight enclosure, afirst screen therein responsive to emit electrons upon incidence of aphoton of input radiation thereon, a second screen therein responsive toincidence of electrons to emit an output radiation to which said firstscreen is photoelectrically responsive, and a circuit connecting saidfirst screen to said second screen and containing means to interruptcurrent flow therein upon passage of a predetermined quantity ofelectricity.

6. A radiation responsive device comprising a vacuumtight enclosure, afirst screen therein responsive to the incidence of a photon of inputradiation thereon and emitting electrons in response to said inputradiation, a second screen therein responsive to incidence of electronsto emit lfan output radiation to which said first screen isphotoelectrically responsive, a honeycomb structure of ducts positionedbetween and normal to said first and second screens, the internal facesof said ducts exhibiting the property of reflecting said outputradiation and circuit means connecting saidfirst screen to said secondscreen and responsive to the amount of current flow and containing meansto interrupt current ow after passage of a predetermined amount ofcurrent.

7. A radiation responsive device comprising a vacuumtight enclosure, afirst screen therein responsive to the incidence of a photon of inputradiation thereon to cause emission of electrons therefrom, a secondscreen therein responsive to incidence of electrons to emit an outputradiation to which said first screen is photoelectrically responsive, ahoneycomb structure of ducts positioned between and normal to said firstand second screens, said ducts having surfaces which freely emitsecondary electrons and a circuit connecting said first screen to saidsecond screen and containing means to interrupt current fiow thereinupon passage of a predetermined quantity of electricity.

8. A radiation responsive device comprising a vacuumtight enclosure, afirst screen therein responsive to the incidence of a photon of inputradiation thereon to cause emission of electrons, a second screentherein responsive to incidence of electrons to emit an output radiationto which said first screen is photoelectrically responsive, a honeycombstructure of ducts positioned between and normal to said first andsecond screens, said ducts having surfaces which are photoelectricallyemissive and a circuit connecting said first screen to said secondscreen and containing means to interrupt current flow therein uponpassage of a predetermined quantity of electricity.

9. A radiation responsive device comprising a vacuumtight enclosure, afirst screen therein responsive to the incidence of a photon of inputradiation thereon to cause emission of electrons therefrom, a secondscreen therein responsive to incidence of electrons to emit an outputradiation to which said first screen is photoelectrically responsive, ahoneycomb structure of ducts positioned between and normal to said firstand second screens, said ducts are coated with cesiated antimony andcircuit means connecting said first screen to said second screen andcontaining means to interrupt current flow therein upon passage of apredetermined quantity of electricity.

10. A radiation responsive device comprising a vacuumtight enclosure, afirst screen therein responsive to the incidence of a photon Yof inputradiation thereon to cause generation of electrons, a second screentherein responsive to incidence of electrons to emit an output radiationto which said rst screen is photoelectrically responsive, a honeycombstructure of ducts positioned between and normal to said rst and secondscreens, said ducts having surfaces which freely emit secondaryelectrons, and circuit means connecting said first and second screens toaccelerate electrons emitted from said first screen and the surface ofsaid ducts into incidence with said second S Creen.

References Cited in the le of this patent 8 Bitner June 14, 1938 IamsSept. 3, 1940 Iams Sept. 16, 1941 Green Dec. 25, 1951 Greenwood et a1July 29, 1952 Marshall et al Sept. 30, 1952

